1. Field of the Invention
The invention is directed to a method and an apparatus for image recording of an object from a flying object by optically scanning the object.
2. Description of Related Art
In the course of the last two decades, aerial photography of, for example, the surface of the earth with the help of conventional photo cameras adapted to this specific purpose has been increasingly replaced or supplemented by high resolution scanning processes, beginning with the help of vidicons, and later with photo-semiconductor line sensors of the charge-coupled memory type. Herein, a flying object, for instance an earth satellite, flies at high altitude over the object, for instance the earth surface, and flies essentially parallel to the surface of the earth. The object is then optically scanned in lines oriented essentially transverse to the direction of flight at a specific angle of view .alpha., which lies in a vertical plane through the direction of flight. The recorded image lines are subsequently processed into an overall image.
The resolution of the image recorded transverse to the direction of flight depends, among other things, on the focal length of the scanner used. Therefore, by increasing the focal length, the ground pixel can theoretically be made as small as desired, even at great flight altitudes, as they occur in the course of imaging the surface of the earth by satellites. On the other hand, the pixel resolution in direction of flight is also dependent on the flight speed and the exposure time. In the case of satellites, the speed of flight is a predetermined, substantially invariable magnitude of approximately 7000 meters-per-second. In order to achieve a sufficiently small pixel resolution in the direction of flight, the exposure time for one line must thus be sufficiently small, or, accordingly, the line frequency must be sufficiently large. However, the limited sensitivity of the sensors used hinders very short exposure times, resulting in the signal-to-noise relationship becoming unfavorable. The same considerations apply also for rapidly moving flying objects in ground proximity, if one requires a large resolution.
For map making imaging of the surface of the earth from cosmic space, a high ground pixel resolution and thus small ground pixel diameters are desired. In order to solve this problem in view of the above difficulties, so-called TDI sensors, where TDI stands for "time delay and integration", have been developed. These sensors consist of several sensors lines arranged in parallel fashion, whose weak image signals are integrated from line to line. These sensors are, however, very expensive. Apart from that, the sensor lines must be very accurately aligned with the direction of flight, and the timing frequency of the line sensors must be exactly tuned to the image movement, i.e. to the speed of flight and the flight altitude of the flying object. Additionally, the sensitivity of the TDI sensor increases not proportionally with the number of the parallel lines, but rather only approximately proportionate to the root, since the noise also increases with the root.
A comprehensive presentation of the digital imaging technology with line sensors can be had from the paper of O. Hofmann, which was published in the publication Bildmessung und Luftbildwesen, 50th year (1982), Vol. 1, lines 16-32.